I've dealt with about 4 cases that had to do with Windows Firewall blocking communication to the server. First thing I would try is to close down the server then follow the instructions below:
Okay so here's a guide for all versions of Windows. Keep in mind I do not recommend keeping Windows Firewall disabled, we're just testing it to see if that's what's blocking access.
How to Disable the Windows Firewall Once the firewall is off you'll need to see if someone can connect.
If it does work then do this: Go back to the general firewall settings turn on the firewall and then on the left you should see "allow an app through the firewall". Click it. Now click the "Allow Another App" button and browse to the TerrariaServer file. Then make sure you can still connect.